Osteoarthritis (OA) is a degradative joint disease affecting millions of people in the US, but there is currently no cure or effective treatment whic targets the disease process. Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine that has been studied for its role in the immune system and in autoimmune diseases including rheumatoid arthritis and systemic lupus erythematosus but has not been previously studied in OA. The objective of the proposed project is to determine the ole of MIF in the development of OA. Our novel preliminary data indicate that 12- and 22-month old MIF knock-out mice develop less age-related OA than their age- and strain-matched controls. Here, we propose two specific aims using both in vitro and in vivo studies to identify the mechanism by which MIF contributes to the development of OA. In Aim 1, we will determine if the deletion of MIF reduces the severity of OA in an animal model of injury-induced OA. We hypothesize that MIF knock-out mice will develop less severe injury-induced OA than their age- and strain-matched controls. We will use the destabilized medial meniscus (DMM) model in both the total MIF-/- as well as a cartilage-specific MIF-/- to compare the systemic and local effects of MIF deletion on the development of injury-induced OA. We will also use histological techniques to analyze the effects of matrix degradation and to compare the macrophage populations in both the aged populations (12- and 22-month) and injury-induced OA populations. In Aim 2, we will use in vitro techniques to culture and stimulate chondrocytes, meniscal cells, and synovial fibroblasts with MIF, followed by protein and RNA analysis. We hypothesize that MIF will activate the MAP kinase signaling pathways, resulting in the increased production of matrix-degrading enzymes and inflammatory cytokines. These studies will allow us to identify signaling pathways activated by MIF in joint tissues as well as analyze the effects of the deletion of MIF on the development of OA. The results fro these proposed studies examining the role of MIF signaling in the development of OA will significantly impact the field by identifying a novel potential therapeutic target in the treatmen of OA.